1. Field of the Invention
The present invention relates to a display device, and more particularly, to a backlight unit for the display device.
2. Discussion of the Related Art
Until recently, cathode-ray tubes (CRTs) have been utilized as display devices. Presently, much effort has been made to study and develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs), as substitutes for CRTs. Among those flat panel displays, the LCD device has many advantages, such as high resolution, light weight, thin profile, compact size, and low power consumption.
In general, the LCD device includes two substrates that are spaced apart and face each other, and a liquid crystal material interposed between the two substrates. The two substrates are provided with electrodes that face each other such that a voltage applied between the electrodes induces an electric field across the liquid crystal material. In accordance with an intensity of the induced electric field, alignment of liquid crystal molecules in the liquid crystal material changes into a direction of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device displays images by varying the intensity of the induced electric field. Unlike the other types of display devices, the LCD device is a non-luminous display device and cannot display images without a light source (e.g., ambient light or a backlight).
A backlight unit for the LCD device may be either a direct type or an edge type. The direct type backlight unit includes a plurality of lamps directly disposed below a light guide plate and thus emits light to an entire surface of a liquid crystal panel. However, the direct type backlight unit has disadvantages of high power consumption and thick profile. On the other hand, the edge type backlight unit includes a lamp arranged along a side surface of the light guide plate. The lamp for the edge type backlight unit may be a cold cathode fluorescent lamp (CCFL). Accordingly, the edge type backlight unit has advantages of thin profile, low weight and low power consumption. The lamp for the edge type backlight unit may be designed to be a straight shape, an “L” shape or a “U” shape.
FIG. 1 is an exploded perspective view of an edge type backlight unit for a liquid crystal display device according to the related art. As shown in FIG. 1, the edge type backlight unit includes a straight-shaped lamp 10, a lamp guide 12, a light guide plate 4, a reflector 2, a prism sheet 6 and a diffusion sheet 8. The lamp 10 is disposed along a side surface of the light guide plate 4. The lamp guide 12 covers and protects the lamp 10. The light guide plate 4 transforms a light emitted from the lamp 10 into a plane light. The light guide plate 4 is formed with a first prism pattern on its bottom surface to increase brightness. The prism sheet 6 is disposed on the light guide plate 4 and is formed with a second prism pattern perpendicular to the first prism pattern. The reflector 2 is disposed below the light guide plate 4 and reflects a light toward the light guide plate 4. The diffusion sheet 8 makes an intensity of the light incident thereto uniform.
FIG. 2 is an exploded perspective view of another edge type backlight unit for a liquid crystal display device having an L-shaped lamp according to the related art. As shown in FIG. 2, the edge type backlight unit includes an L-shaped lamp 34, an L-shaped lamp guide 36, a light guide plate 24, a reflector 22, first and second diffusion sheets 26a and 26b, and first and second prism sheets 28a and 28b. 
The L-shaped lamp 34 is disposed along two adjacent side surfaces of the light guide plate 24. The L-shaped lamp guide 36 covers and protects the lamp 34. The light guide plate 24 transforms a light emitted from the lamp 34 into a plane light. The light guide plate 24 is formed with a diffusion pattern on its bottom surface. The diffusion pattern makes the light incident to the light guide plate 24 emitted toward a liquid crystal panel. The first and second diffusion sheets 26a and 26b make an intensity of the light incident thereto uniform. The first and second prism sheets 28a and 28b are formed with first and second prism patterns, respectively, perpendicular to each other. The reflector 22 reflects a light toward the light guide plate 24.
As described above, the backlight unit of FIG. 2 includes the light guide plate 24 having the diffusion pattern, and the two prism sheets 28a and 28b that change light progressive path vertically and concentrate light. Accordingly, the backlight unit of FIG. 2 has a complex structure, which brings about an increase in the number of fabrication processes and fabrication costs. To resolve these problems, the backlight unit of FIG. 2 may utilize the light guide plate (4 of FIG. 1) having the prism pattern of the backlight unit of FIG. 1. However, since the light emitted from a portion of the L-shaped lamp 34 is perpendicular to furrows of the prism pattern of the light guide plate of FIG. 1, part of the light from the portion of the L-shaped is not incident to the light guide plate of FIG. 1 and therefore is wasted. In other words, the portion of the L-shaped lamp 34 parallel to the furrows of the prism pattern of the light guide plate of FIG. 1 emits a light perpendicular to the prism pattern, thereby causing a loss of the part of the light incident to the light guide plate.
Accordingly, when utilizing the light guide plate having the prism pattern instead of the diffusion pattern, the backlight unit of FIG. 2 has light efficiency less than the backlight unit utilizing the diffusion pattern. In addition, an undesirable bright line is generated.